For drilling an oil field or a natural gas field, oil country tubular goods are used. In proportion to the depth of a well, multiple steel pipes are coupled to form the oil country tubular goods. Coupling the steel pipes is conducted by fastening threaded connections for pipe or tube formed at end portions of the steel pipes. The steel pipes are drawn up and loosened for inspection or the like, and after the inspection, the pipes are fastened again and reused.
A threaded connection for pipe or tube includes a pin and a box. The pin has, on an outer peripheral surface of a front end portion of a steel pipe, a contact surface including an external thread portion and an unthreaded metal contact portion. The box has, on an inner peripheral surface of a front end portion of a steel pipe, a contact surface including an internal thread portion and an unthreaded metal contact portion. The thread portions and the unthreaded metal contact surfaces of the pin and the box repeatedly receive strong friction in fastening (making-up) and loosening (breaking-out) steel pipes. If these portions do not have a sufficient durability against friction, galling (unrepairable seizure) occurs when fastening and loosening are repeated. Therefore, a sufficient durability against friction, namely, excellent galling resistance is needed for a threaded connection for pipe or tube.
In conventional practices, to improve galling resistance, a compound grease called dope, containing heavy metals, has been used. By applying the compound grease on the surface of a threaded connection for pipe or tube, the galling resistance of the threaded connection for pipe or tube can be improved. However, heavy metals such as Pb, Zn, and Cu contained in the compound grease may exert an influence on the environment. For this reason, the development of a threaded connection for pipe or tube containing no compound grease has been desired.
Japanese Patent Application Publication No. 2002-221288 (Patent Literature 1) and International Application Publication No. WO2009/072486 (Patent Literature 2) propose threaded connections for pipe or tube excellent in galling resistance even without compound grease.
On the contact surfaces of the pin and the box of a threaded connection for pipe or tube described in Patent Literature 1, a porous Zn layer or a porous Zn alloy layer is formed on at least one of thread portions or unthreaded metal contact portions of the threaded connection for pipe or tube by a mechanical plating method, and on the layer, a solid lubricant coating or a liquefied lubricating coating containing no heavy metal powders (e.g., a coating containing a high-basic organometallic salt of a high-basic sulfonate or the like as its main agent) is formed. Patent Literature 1 describes that it is thereby possible to have a high anticorrosion property and suppress the occurrence of galling due to occurring or the reduction of gas-tightness at the time of repeating fastening/loosening while dispensing with-liquid lubricant, such as compound grease, containing heavy metal powders.
In a threaded connection for pipe or tube described in Patent Literature 2, the contact surface of a box includes, as an uppermost layer, a solid lubricant coating having plasticity or viscoplastic rheological behavior, and the contact surface of a pin includes, as an uppermost layer, a solid anticorrosive coating containing a UV cured resin as a main constituent. Patent Literature 2 describes that it is thereby possible to obtain a threaded connection for pipe or tube that suppresses the occurrence of rust without using compound grease, exerts excellent galling resistance and gas-tightness, has no stickiness on its surface, and is excellent in appearance and inspectability.